Telephone and related communication companies transmit electrical signals through a plurality of communication conductors that are bound together and housed within a cable sheath. To maintain the conductors in a dry condition, the sheath is pressurized with a low humidity gas, i.e., nitrogen, or dry air supplied from a compressor dehydrator. The gas or dry air (herein after referred to as "air") is introduced into the sheath at approximately ten pounds pressure. If a sheath leak occurs, the outward pressure of the air prevents the inward penetration of moisture into the sheath interior. If the leak can be located within a reasonable time, the entry point of the moisture can be repaired and the dry and dehumidified air ambient interior of the individual communication conductors preserved.
A typical pressurization system includes a central office compressor-dehydrator that supplies dehumidified air via a B-meter panel to individual cable sheaths and via a pipe-alarm panel to an air pipe system that supplies the air to a series of manifolds located in a manhole run. Each manifold has five outlets, with each outlet pressurizing an individual cable sheath enclosing 900 to 3600 paired conductors. The pressure in each sheath is remotely measured at the central office by utilizing pressure transducers that are placed at approximately 6000 foot intervals preferably between manifolds.
The current method used to locate cable sheath air leaks is to remotely detect and measure the pressure drop and the air-flow rate in the pressurized cable. Measurements have been tried from both the central office meter panels and at the individual manhole locations where the manifolds are located.
Measurements taken from the central office can only grossly evaluate an entire cable section and is of little or no help in locating or evaluating, with any precision, the location of the leak beyond the first manifold.
Measurements taken at the individual manholes are accomplished by sending maintenance technicians to the suspected manhole location. The technicians are required to (1) open and enter the manhole, (2) take pressure readings with a pressure gauge, (3) measure the flow rate with a portable flow rater, (4) determine direction of flow with a directional flow indicator and (5) proceed to adjacent manholes to take additional readings. Considerable time is used and readings from one manhole to another are often confused and/or lost. The end result is that individual sheath leaks in many cases cannot be satisfactorily located within a time interval to permit a repair that will prevent an interruption of communication.
A search of the prior art did not disclose any patents or publications that were directly related to the instant invention. However, the following U.S. patents were considered in the investigation and evaluation of the prior art.
______________________________________ PATENT NUMBER INVENTOR ISSUED ______________________________________ 4,201,079 Worcester 6 May 1980 4,007,628 Worcester 15 February 1977 3,964,292 Jackson 22 June 1976 ______________________________________
The Worcester patents disclose a method and an apparatus for remotely monitoring gas leakage in communication cables. The invention consists of attaching, in series with a manhole manifold, a volume flow meter. The meter can be remotely sampled to provide an indication of the gas flow through the meter. Through a network of such meters, in combination with conventional pressure transducers, cable gas leaks can be detected, measured, located and repaired.
The Jackson patent discloses an apparatus and method for detecting a gas leak occurring at any point along a path. A tubing having a wall through which the gas readily permeates is provided. This tubing is structured so that the gas can enter the tubing only by permeating the tubing wall. The gas detection and approximate location of permeation is achieved by withdrawing the gas within the tubing while monitoring both the withdrawn gas and the elapsed time.